1. Field of the Invention
The field of the present invention is mixing silos for free-flowing finely divided solid materials, in particular for powdered, fibrous and/or granular mixed material, especially polymer granules, specifically suited for mixing polymer granules, particularly, mixing silos for homogenizing possibly inhomogeneous polymer granule batches in the form of a stream of product from a process producing polymer granules.
2. Background
In mixing operations of mixing silos, additional finely abraded material should be avoided as far as possible, and there should be no foreign contamination by remains of granules from previous mixing and filling processes of other types of granules in the silo.
The granular product occurring in the reactor during the production of thermoplastics is plasticated in an extruder and formed in a granulating die into individual strands, which are cut into granules by means of a knife rotating in the granulating die. This product may be provided with further components in a further step by compounding.
Compounding is the term used in polymer preparation for producing the finished moulding compound from the raw plastics materials with the addition of fillers, reinforcing agents, plasticizers, coupling agents, lubricants, stabilizers, etc. The compounding is mainly performed in extruders and comprises the process operations of conveying, melting, dispersing, mixing, degassing and building up pressure.
During the granulation, the melt is then forced through the orifices of a die plate, so that subsequently, in the case of strand granulation, initially melt strands are produced, and then during the granulation these strands are turned into cylindrical granules, or else, in the case of die-face granulation, the strands are cut off directly as they emerge at the die plate and are turned into lenticular or spherical granules. The granulation may be performed, for example, in a stream of liquid, which cools the granules and largely avoids agglomeration. The granules are subsequently dried and screened, in order to separate out agglomerates formed in spite of cooling.
Following the granulation after production or after compounding, the product is generally conveyed pneumatically to a mixing silo.
In the mixing silo, the granules are homogenized to balance out fluctuations in the production process, and are possibly subsequently transported pneumatically into the storage silos.
The mixing silos known from the prior art are generally operated as gravity mixers or circulating mixers. For both types there are numerous proposals in the prior art as to how to use suitable internals in the silo container to achieve a high mixing quality, that is to say good homogenization of different bulk materials that are usually introduced into the silo container one after the other, even after the bulk material has passed through only once, or—in the case of circulating mixers—to keep down the number of circulations, and consequently the mixing time.
Depending on the requirement and design, an acceptable mixing quality is accordingly already achieved upon discharge. DE 41 12 884 C2 gives in the background description a comprehensive overview of the prior art, which is based substantially on the installation of funnel-shaped internals in the conical region of the mixing silo.
A disadvantage of the subject matter of DE 41 12 884 C2, however, is that a separate, multi-chamber funnel has to be fitted in the bottom region of a silo container, which makes it much more complicated to produce and maintain the mixing silo, since such a construction is quite difficult to clean.
A different approach to a solution is taken by the so-called in-line mixers, where vertical pipes with intake openings internally in the silo pass granules to the outlet from various heights. For example, multi-pipe blenders are used, in which the tubular channels on the inner wall are arranged into the conical region and achieve a degree of mixing while the material is simply running out. A disadvantage is the design-related effort that is correspondingly required for cleaning with water to avoid contamination.
The aim of all the configurations is to achieve an acceptable mixing quality with the lowest installation and operating costs and the easiest cleaning.
If the prior art is analysed with a view to this, it discloses strengths and weaknesses which have been sufficiently well documented in the literature.
For example, DE 12 98 511 and EP 60 046 A1 each show a mixing silo, the interior space of which is subdivided by vertical sheet segments, extending radially from the container outer wall to the centre axis thereof, into a number of chambers, which, given a suitable position of the filling opening, fill one after the other in accordance with the overflow principle as a result of correspondingly staged upper edges of the sheet segments, whereby often a vertical pre-mixing—albeit dependent on the batch size—is achieved instead of the purely horizontal layering that otherwise occurs.
A disadvantage of DE 41 12 884 C2, DE 12 98 511 and EP 60 046 A1 is, however, that the inlet and outlet cross sections of the mixing cross are each formed approximately the same, and consequently only a limited mixing-through of the bulk material can take place, with at the same time poor cleaning possibilities to avoid cross contamination.
Furthermore, DE 22 19 397 already discloses a mixing silo designed as a circulating mixer, in which the central rising pipe is surrounded by a further, comparatively much shorter pipe, so that this further pipe defines with the central pipe a first annular space and with the silo container wall and the conical bottom thereof a second annular space. During the circulation or removal of the bulk material, different sinking rates of the bulk material are obtained in the two annular spaces, so that fractions of bulk material originating from levels at different heights are blended or mixed with one another in the outlet region. Also based on a similar principle is the gravity circulating mixer known from DE 30 29 393 A1, in which, however, the circulation is not performed by way of a central pipe but by way of a vertical rising pipe running outside the silo container.
DE 21 58 579 A1 discloses a device for the continuous mixing of granular solid materials, which construction, in particular due to the valves (separately adjustable metering device) within the silo as well as due to the partly filled hopper leads to cross contamination during process. Similar apparatuses are described in JP 56 111028 A and JP 59 053836 U in which the individual chambers can be closed off individually and according to the construction the apparatus does lead to cross contamination during operation.
These known mixing silos all have the disadvantage that the internals provided in the silo container are subjected to considerable static and dynamic loads. Although the known mixing silos are designed without exception for mass flow conditions, there are also deposits of bulk material, which make it much more difficult for the silo container to be washed out. The requirement for the silo container to be suitable for washing out easily before filling with another type of bulk material is however being demanded increasingly frequently.